1. Field of the Invention
The present invention relates to an electric tool, and in particular to a manually operated electric driver for clamping a screw or a nut.
2. Description of the Prior Art
As an electric tool for clamping a screw or a nut is known, an electric driver which, as shown in FIG. 1, includes a direct-current motor C with a brush B as a drive source for rotating a bit A. An important point relates to the need for an electric driver whose rotational speed for the screw-clamp is variable in accordance with the kind of an object to be clamped by a screw. For example, when clamping a tapping screw on a resin member, a speed which is too high results in an insufficient clamp of the screw due to the melt of the resin member, whereas the a speed which is too low causes a decrease in work efficiency. Furthermore, when clamping a tapping screw on a wooden member, the too low screw speed causes a crack on the wooden member, while the too high screw speed causes an unsatisfactory clamp because of breakage of the clamped portion of the wooden member.
For setting the screw clamping speed in accordance with the kind of object to the conventional electric driver, a special device is required, for example, detecting the rotational speed of the motor, resulting in an increase in cost.
A unique problem with the dc motor is the limited life of a brush B and the generation of dust. An exchange of the brush B should be required whenever its operating time reaches 300 to 500 hours and the generation of dust results in unsuitability for the use in a clean room.
A further problem associated with such a conventional electric driver including the dc motor C with the brush B is that a rotor D with the winding having a greater number of turns should be used for the purpose of producing a sufficient rotational torque. This results in a bulky rotor, increase in weight, and significant increase in moment of inertia of the rotor D. Therefore, the variations of the motor speed lead to the variations of the driving torque. In addition, because of the relationship between the speed and the torque, i.e., speed.times.torque=constant value, the torque is varied in response to the change of speed, that is, difficulty is encountered in maintaining an appropriate screw clamping torque.